Objectives The crop production in Hainan Island is challenged by excessive nitrogen fertilizer input, high nitrogen loss, and low nitrogen utilization efficiency. Guided by the principle of nutrient balance, we determined the maximum nitrogen fertilizer input for major crops and the regional nitrogen carrying capacity, ensuring a balance between increased crop yields and environmental safety. Additionally, we will propose optimization measures for nitrogen fertilizer management.

Methods We conducted an extensive survey among farmers to determine crop yield potential. Following this, we established databases of nitrogen concentrations in crop products and optimal nitrogen surplus levels under optimal crop management conditions in South China. These databases were meticulously crafted using self-measured data, information from the Food and Agriculture Organization (FAO) statistics, and published literature. Subsequently, we evaluated nitrogen requirements, surplus thresholds, and nitrogen fertilizer quotas under conditions conducive to high yields, all based on the principle of nutrient balance. Additionally, we quantified Hainan’s regional-scale nitrogen capacity by integrating this information with the local cropping structure.

Results The average yields of grain (rice, for example) and vegetable crops in Hainan were 6.2 and 33.9 t/hm², which were 13% and 32% lower than the national yield, respectively. However, the average yield of fruit crops was 33.6 t/hm², which was consistent with the national yield level. The nitrogen surplus thresholds for grain, vegetable, and fruit crops in South China were 81 kg/hm², 108 kg/hm² and 283 kg/hm², respectively. The average nitrogen application quota for grain crops was 160 kg/hm², which represented a potential reduction in nitrogen application of 21%, compared to current levels. For vegetable crops, the nitrogen application quota ranges from 181 kg/hm² to 476 kg/hm², with a general potential reduction of 22%, and the greatest potential reduction of 39% in legume crops. The nitrogen application quota range for fruit crops was 240−689 kg/hm², with a potential reduction of 28%. The nitrogen application quota range for other economically important crops was 196−306 kg/hm². The regional nitrogen carrying capacity of farmland in Hainan cities and counties ranged from 4 kt to 28 kt, amony them, Ledong had the highest capacity (28 kt), Baisha and Wuzhishan had the lowest carrying capacity (4 kt). Currently, the region’s nitrogen input is 33% higher than the carrying capacity, but the output is 48% lower than the target, the nitrogen surplus exceeds the target by 156%, and the current nitrogen use efficiency is 23%, which is lower than the target by 37 percentage points.

Conclusions The yield level of Hainan’s grain and vegetable crops is lower than the national yield level, but the yield level of fruit crops is in line with the national yield level. The high nitrogen surplus thresholds for economic crops result in elevated nitrogen fertilizer quotas, primarily because the current nitrogen fertilizer management for economic crops does not fully implement the optimization of all principles. Hainan’s current regional nitrogen input has exceeded its safe carrying capacity, particularly in cities and counties with a high proportion of economic crops such as Haikou, Lingao, Danzhou, and Wanning. In the future, nitrogen fertilizer management in Hainan should prioritize researching techniques to reduce nitrogen intensity and enhance efficiency especially for economic crops, while also optimizing their nitrogen surplus thresholds and fertilizer quotas, and the proportion of organic fertilizers replacing chemical fertilizers should be increased.